The present invention relates to a method for the preparation of hydroxybenzoic acid. More particularly, the invention relates to a method for the preparation of hydroxybenzoic acid, such as 4-hydroxybenzoic acid which is a useful starting material in the manufacture of various chemical products including antiseptic agents, fungicidal agents, high-polymeric liquid crystals, color-developing agents on heat-sensitive recording paper and so on, in which the alkali metal hydroxide used in the reaction can be efficiently recovered in the form suitable for re-use containing little impurities.
As is known, the most conventional prior art method for the preparation of hydroxybenzoic acid is to utilize the socalled Kolbe-Schmitt reaction. In this method, potassium salt of phenol, i.e. potassium phenolate, is heated in the presence of carbon dioxide gas to form hydroxybenzoic acid which is then precipitated in and isolated from the reaction mixture by acidification with hydrochloric acid.
One of the problems in this prior art method is that the mother liquor in the precipitation of the hydroxybenzoic acid by acidification contains potassium chloride which is the reaction product of hydrochloric acid with potassium hydroxide used as one of the starting materials and the potassium chloride contained in the mother liquor can be recovered only with great difficulties because the mother liquor necessarily contains considerable amounts of phenol, hydroxybenzoic acid, by-products and other organic materials.
Needless to say, it is very important from the standpoint of economy in the industrial production of hydroxybenzoic acid by the above described process to recover the potassium chloride from the mother liquor and convert it by electrolysis into potassium hydroxide to be re-used. Several attempts and proposals of course have been made hitherto in this regard but none of them is suitable for industrialization.
For example, Japanese Patent Publication No. 49-48304 teaches a method for the recovery of the potassium component in the above described process, according to which the acidified mother liquor is reacted with chlorine and freed from the thus formed hardly soluble or insoluble chlorinated compounds followed by electrolysis. This method, however, has several disadvantages that the use of toxic and corrosive chlorine necessarily causes problems in the workers' safety and corrosion of apparatuses, that the chlorinated compounds produced by the reaction of chlorine also have toxicity and that sufficiently high recovery of the potassium component can hardly be obtained. Therefore, this method has only a very limited practical value.